Quote:
Quote:
Take two grains of salt and call me in the morning.
The only way to do an accurate comparison of BC is to shoot drop tests at long range.
Chronographs are not accurate enough from round to round for a comparison like that.
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I hear ya...
I had a "Come to Jesus" meeting with a bunch of chronographs about five years ago.
About 10 years ago, I was talking to Ken Oehler and we got on the subject of how do you calibrate a chronograph.
I mean, suppose your chrono says your bullets are doing 4,000 fps.
How do you know that it is 4,000 - might be 3940 or 4085, or whatever - kinda like calibrating your car's odometer for error with 10 miles worth of highway markers...
Since shooters have complete, total, God-like trust in their chrono (even if it cost $29.95), I thought it might be interesting to find out how trustworthy these things are.
All the Chrono's use the same $2 clock chip and it runs at 4 megacycles - so if it is off, that might be a source of error - then you could adjust the trap spacing to make up for the clock error - that was my thinking at the time...
... after about 45 minutes of brain storming, we came to the conclusion that there was no practical way to calibrate it, and you just took the numbers from the screen and that was what you got, +/- some unknown... and you had a very good idea of the speed, but not down to the foot (would that only be true).
End of chapter one.
Chapter two
I (like many others) from time to time had odd readings from my chrono - like one day when I shot one of the smallest groups I had ever shot, and the ES was 130 (/ubbthreads/images/graemlins/confused.gif) and other times when I was working up loads with known stable combinations, and the ES would go from 8 to 47 to 21 to 51 to 13 to... whatever???
So, on a whim, about 5 years ago, I lined up 4 Chronos in a line - each with a 4ft rail, and 5 feet apart.
The four were a Oehler 33, an Oehler 35P, a CED, and a Pact.
Care was used with a setup jig so that the bullet would fly 2" over the light port on every trap.
The rifle was a proven long range benchrest rifle whose loads were stable, and based on the groups at 600 yds, the ES was dammn tiny.
Based on the spacing of the different rails, the difference in the chrono readings should have been 7.5 fps, so if the first one said 3,500, the next ones should 3,492.5, 3,485, 3,477.5, and 3,470.
That's easy!
On the first shot, the numbers were all over the place. I looked at them and it was clear that there was no steenkin 7.5 fps difference - there was ~85 fps from the highest to the lowest, and the others were spread all over.
So I figured that the errors were going to be like the first range of spreads.
I fired the second shot, and the numbers were all over the place in a different order /ubbthreads/images/graemlins/frown.gif
So I fired the next three shots and looked at the summaries.
None of the chronos showed the same ES, AV, or SD.
I repeated the same test with the same load from the same box - and none of the chronos showed the same numbers, or even the same patterns - there was a very large randomness to the whole pool of data.
It took a year to figure where the bulk of the errors are coming from, but the point of this is you cannot count on a chrono to detect small differences in BC - you MUST do long range drop tests.
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